Foamed adhesives show improved economics and performance over adhesives which are not foamed. Since foamed adhesives do not penetrate porous surfaces to the same extent as non-foamed adhesives, the open time increases and the tendency for pre-cure decreases. In addition, at any given film thickness, a foamed adhesive contains less water than an unfoamed adhesive. Less water in the adhesive leads to little or no grain raise, and improved non-curl. Foamed adhesives also have the ability to hold out on a substrate surface to a greater extent than unfoamed adhesives, resulting in less wasted adhesive.
Synthetic resin-based foamable adhesives are known in the art. U.S. Pat. No. 4,561,918 describes a foamed adhesive of an ethylene/vinyl acetate copolymer. U.S. Pat. No. 5,696,174 describes a method for producing a foam having increased stability, by using a composition having both a long-chain organic cation-forming compound, and a long-chain organic anion-forming compound. Foams produced from synthetic polymers and natural rubber emulsions are described.
JP 57143370 describes an aqueous adhesive in which air bubbles are mixed with a synthetic polymer or starch by mechanical stirring or chemical reaction. No surfactant is described. JP 62064877 describes a foamed aqueous adhesive having a foam stabilizer surfactant. The adhesive polymer is listed as either a water soluble resin (starch, cellulose derivatives, gelatin and polyvinyl alcohol) or a water dispersible synthetic resin. All of the examples are directed to synthetic resins.
While methods have been proposed for the foaming of any polymer adhesive, both synthetic and natural, success has been demonstrated only with synthetic polymers. Foamed polysaccharide-based adhesives tend to be unstable and dissipate within a few minutes. There has been a long-felt need to find a process for foaming a natural (polysaccharide-based) aqueous polymer adhesive which would provide a highly stable, consistent foam. Surprisingly, the process of the present invention produces a highly stable and consistent aqueous foamed adhesive. The high stability and consistency of the foam allow its use in manufacturing applications requiring extended usage times. The foamable water-based adhesive produced by the process of the present invention offers a natural-based alternative to synthetic-based foamable adhesives currently on the market.
The problem with foamed dextrin or starch based adhesives is that the foam produced is unstable and dissipates within minutes. The addition of a filler allows for foam generated to remain consistent and stable for several hours, which is necessary in many cases to allow for extended usage time in many applications.